Method for temperature calibration of blade strain gauges and wind turbine rotor blade containing strain gauges

ABSTRACT

A method for temperature calibration of a strain sensor for a rotor blade of a wind turbine is provided, the method including operating the wind turbine in a mode in which substantially no bending of the rotor blade due to wind occurs, repeatedly measuring gravitationally induced bending moments of the rotor blade for a plurality of temperatures measured at the place of the strain sensor, determining a temperature dependency of the strain sensor on the temperature, calibrating the strain sensor based on the determined temperature dependency of the strain sensor such that the temperature dependency of the strain sensor is compensated.

BACKGROUND OF THE INVENTION

The present disclosure relates to wind turbines. Particularly, thepresent disclosure relates to a wind turbine rotor blade which isadapted to perform a temperature calibration of a strain sensor andmethods for temperature calibration of a strain sensor arranged at arotor blade of a wind turbine.

Although horizontal axis wind turbines are well-established these days,there is still considerable engineering effort going on to furtherimprove their overall efficiency, power generating capability, androbustness.

Modern wind turbines are designed to produce a maximum amount of energyin a particular geographical area. Therefore, wind turbines are operatedsuch that the operational wind speed range is increased. This increasesthe loads on almost all parts of a wind turbine, especially the rotorblades which are typically produced from light weight materials, likeglass or carbon fibers. Excessive loads will result in fatigue failuresof the rotor blades. As the power generation should be maximized, windturbine rotor blades are operated closer and closer to their fatiguelimit.

As fatigue failure of rotor blades should be avoided, there is a need toknow exactly when those fatigue failures will occur. Typically, fatiguefailures will occur at a well-defined stress within the material of therotor blades. As the material constants for the material used to buildthe rotor blade are known, one can predetermine the force at which thematerial will break.

BRIEF DESCRIPTION OF THE INVENTION

In view of the above, a rotor blade including a first strain sensorarranged at a surface of the rotor blade, and a first temperature sensorarranged adjacent to the first strain sensor is provided.

According to another aspect of the disclosure, a method for temperaturecalibration of a strain sensor arranged at a rotor blade of a windturbine is provided. The method includes operating the wind turbine in amode in which substantially no bending of the rotor blade due to windoccurs, repeatedly measuring gravitationally induced bending moments ofthe rotor blade for a plurality of temperatures measured at the locationof the strain sensor, determining a temperature dependency of the strainsensor from said measured data, calibrating the strain sensor based onthe determined temperature dependency of the strain sensor such that thetemperature dependency of the strain sensor is compensated.

According to yet another aspect of the disclosure, a further method fortemperature calibration of a strain sensor arranged at a rotor blade ofa wind turbine is provided. The method includes controlling atemperature of a part of the rotor blade in which said strain sensor islocated, measuring a strain using the strain sensor, measuringtemperature at the location of the strain sensor, varying the controlledtemperature and repeating the strain and temperature measurements at adifferent temperature; determining a temperature dependency of thestrain sensor from said measured data, and calibrating the strain sensorbased on the determined temperature dependency of the strain sensor suchthat the temperature dependency of the strain sensor is compensated.

Further aspects, advantages and features of the present invention areapparent from the dependent claims, the description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure including the best mode thereof, to oneof ordinary skill in the art, is set forth more particularly in theremainder of the specification, including reference to the accompanyingfigures wherein:

FIG. 1 is a schematic side view of a wind turbine having rotor bladesaccording to embodiments described herein.

FIG. 2 is a schematic drawing of a Wheatstone bridge circuit used inembodiments described herein.

FIG. 3 is a schematic drawing illustrating four strain gauges which areelectrically connected to each other for forming a Wheatstone bridgecircuit used in embodiments described herein.

FIGS. 4, 5, 6, and 7 are schematic longitudinal cross sectional views ofa wind turbine rotor blade according to embodiments described herein.

FIG. 8 illustrates a method for temperature calibration of a strainsensor arranged at a rotor blade of a wind turbine according toembodiments described herein.

FIG. 9 illustrates the relationship between measured blade moment curvesand the parameters ratio of span and difference of offset which are usedin embodiments described herein.

FIG. 10 illustrates a further method for temperature calibration of astrain sensor arranged at a rotor blade of a wind turbine according toembodiments described herein.

FIG. 11 is a schematic longitudinal cross sectional view of a rotorblade root of a wind turbine according to embodiments described herein.

FIG. 12 is a schematic longitudinal cross sectional view of a rotorblade root of a wind turbine according to embodiments described herein.

FIG. 13 is a schematic perspective view of a rotor blade root of a windturbine according to embodiments described herein.

FIG. 14 is a schematic longitudinal cross sectional view of a rotorblade root of a wind turbine according to embodiments described herein.

FIG. 15 is a schematic perspective view of a rotor blade root of a windturbine according to embodiments described herein.

FIG. 16 illustrates yet a further method for temperature calibration ofa strain sensor arranged at a rotor blade of a wind turbine according toembodiments described herein.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the various embodiments, one ormore examples of which are illustrated in each figure. Each example isprovided by way of explanation and is not meant as a limitation. Forexample, features illustrated or described as part of one embodiment canbe used on or in conjunction with other embodiments to yield yet furtherembodiments. It is intended that the present disclosure includes suchmodifications and variations.

FIG. 1 is a schematic side view of a wind turbine 100. The wind turbine100 includes three rotor blades 110 but may have more or less blades 110according to other embodiments. The rotor blades 110 are mounted on arotor huh 120 which is connected to a nacelle 130. Nacelle 130 is fixedon top of a tower 140. Rotor blades 110 include strain sensors which areadapted to measure bending moments of rotor blades 110.

The rotor blades of a wind turbine are subject to considerable forcesand moments due to wind load, gravitation, and centrifugal force. Suchblade moments are typically proportional to a stretching or bending ofthe rotor blades which can be measured by strain sensors. Typical strainsensors are strain gauges which may be realized as flat electricalresistors. The resistance of those flat resistors changes linearly tolength variations of the sensors. Typically, those strain gauges arebonded or glued onto a surface of the rotor blades being tested. Straingauges may measure tiny relative length variations, as small as 10⁻⁶.

A fundamental parameter of a strain gauge is its sensitivity to strain,expressed quantitatively as the gauge factor (GF). The gauge factor isdefined as the ratio of relative change in electrical resistance (ΔR/R)to the strain which is the relative change in length (ΔL/L).

${GF} = \frac{\Delta \; {R/R}}{\Delta \; {L/L}}$

For metallic foil gauges, the gauge factor is typically about 2.

Typically, strain measurements seldom involve strains larger than 10⁻³.Therefore, typically electrical resistance changes smaller than 1Ω areto be measured. For measuring such small electrical resistances changes,strain gauges are typically used in a Wheatstone bridge configurationincluding a voltage excitation source.

Typically, the resistance strain gauges are bonded to a surface of therotor blade in order to detect the strain in the surface. However,strain gauges exhibit temperature dependency and, therefore, should beoperated such that inaccuracies due to change in temperature arecompensated or at least reduced. This is due not only to the fact thatthe resistance of most of the conductive materials used in bondedresistance strain gauge filaments changes with temperature, but alsobecause the coefficient of thermal expansion (CTE) of the strain gaugefilament is often different from that of the structure to which it isbonded and sensitivity to or measurement of strains resulting fromthermal expansion is not desired and will lead to measurementinaccuracies. Thus, even if the filament of the strain gauge were notdirectly temperature-sensitive because of its change in resistance withchange in temperature, it would still be subject to false strainindications with temperature unless it has a coefficient of thermalexpansion matched with the CTE of the surface to which it is bonded.Such matching is difficult and expensive because a strain gauge matchedto steel is greatly in error if bonded to aluminum or some othermaterial, and vice versa.

Temperature variations cause various effects in strain gauges. Themeasurement object changes in size by thermal expansion, which isdetected as strain. The electrical resistance of the strain gauge andthe electrical resistance of the connecting wires will change. Theresistance changes of the strain gauges are on the same magnitude as thelength changes. Typically, they are both on the order of 10⁻⁶. Tomeasure such small resistance changes, strain gauges are typically usedin a Wheatstone bridge arrangement with a voltage excitation source.

Since temperature variations also result in resistance variations in agauge filament, some means should be employed to cancel the effects ofthese temperature changes from the strain measurements. The resistancevariations which accompany temperature variations are caused not only bythe thermal coefficient of resistivity of the wire filament, but also bythe differences in the linear CTEs of the wire and test structure. Thus,the temperature sensitivity of a particular gauge will vary according tothe material to which it is affixed, and any compensating system must beusable with a variety of test materials. Such temperature sensitivity isoften experienced as “apparent strain” rather than as a variation inresistance, since the ultimate error calculation must relate to theactual strain measurements. The most widely used temperaturecompensation device for strain gauges, the above mentioned Wheatstonebridge, makes use of the electrical system to which they areelectrically connected and also of the mechanical system to which theyare fixed.

Undesired temperature effects may be compensated to a good degree usinga Wheatstone bridge circuit. However, differences due to different CTEsof the strain sensor and the underlying material may not be compensatedfor completely. This can be done by an exigent temperature calibrationin which strain sensors measure strain at various locations of the rotorblade while simultaneously measuring the temperature at those strainsensors.

FIG. 2 shows a schematic drawing of a Wheatstone bridge circuit 190 asit will be employed in embodiments disclosed herein. Wheatstone bridgecircuit 190 includes four resistors 181, 182, 183, and 184 which areconnected to each other to form a closed loop of the four resistors 181,182, 183, and 184 arranged as depicted in FIG. 2. At the junctions ofthe resistors 181, 182, 183, and 184 are provided four Wheatstone bridgecontacts 191, 192, 193, and 194. Typically, an excitation voltage V_(B)is applied between Wheatstone bridge contacts 191 and 194 and a bridgevoltage V is measured between contacts 192 and 193. The bridge voltage Vequals:

$V = {{V_{B}\left\lbrack {\frac{R_{181}}{R_{181} + R_{182}} - \frac{R_{184}}{R_{183} + R_{184}}} \right\rbrack}.}$

Here, R₁₈₁, R₁₈₂, R₁₈₃, and R₁₈₄ are the resistances of the fourresistors 181, 182, 183, and 184. In case the ratio R₁₈₂/R₁₈₁ equals theratio R₁₈₃/R₁₈₄, the voltage V is equal to zero in which case theWheatstone bridge is said to be balanced. Any change in resistance ofany resistor of the bridge results in a nonzero bridge voltage whichtypically is the case if the resistance variations of the four resistors181, 182, 183, and 184 are not equal.

In case of wind turbines, the four resistors 181, 182, 183, and 184 aretypically realized by strain sensors or strain-gauges. There are variouspossibilities to use a Wheatstone bridge with strain gauges. It ispossible to use only one strain gauge as a Wheatstone bridge resistor,in which case the Wheatstone bridge circuit is called a quarter-bridgecircuit. If two strain gauges are used as Wheatstone bridge resistors,the Wheatstone bridge circuit is called a half-bridge circuit. If allWheatstone bridge resistors are strain gauges, the Wheatstone bridgecircuit is called a full-bridge circuit. The measured signal of a fullWheatstone bridge is linearly proportional to the strain to be measured.

Typically, for measuring strain of a rotor blade, two strain sensorsmeasure strain of opposite sign at the rotor blade root. If, e.g., arotor blade root bends towards a definite direction, strain of oppositesign occurs at circumferentially opposite locations of the rotor bladeroot. In case of edgewise bending, the strain sensors are arrangedparallel to the edge-wise direction on opposing inner walls of the rotorblade root. Each strain sensor includes two strain gauges which are bothaligned along a direction from rotor blade root to rotor blade tip. Thestrain gauges of each strain sensor are arranged on opposing sides ofthe Wheatstone bridge circuit 190. In that case, the measured voltage Vis linear proportional to the edgewise strain of rotor blade 110. Inthat case, the compensation of the CTE is realized between the straingauges of the same strain sensor, so mostly at the same location.However, other strain gauge arrangements are also possible where straingauges at different locations of the rotor blade are CTE compensated.

To additionally measure flapwise strain, a further Wheatstone bridgecircuit may be used whose strain sensors are arranged parallel to theflapwise direction.

FIG. 3 is a schematic drawing illustrating four strain gauges which areelectrically connected to each other to form a Wheatstone bridge circuit190. The Wheatstone bridge resistors 181, 182, 183, and 184 are realizedby four strain gauges. The resistors 181 and 183 form a left strainsensor 160 and the resistors 182 and 184 form a right strain sensor 160.The four strain gauges are aligned along a direction from rotor bladeroot to rotor blade tip, the direction being indicated by an arrow inFIG. 3. The left strain sensor 160 is arranged inside the rotor bladeroot circumferentially opposite to the right strain sensor 160. Thus,the left strain sensor 160 measures opposite equal strain compared tothe right strain sensor 160. As the resistors 181 and 183 on the leftare arranged at the same location of the rotor blade root, they aretypically temperature compensated to a good degree. As the resistors 182and 184 on the right are also arranged at the same location of the rotorblade root, typically they are also temperature compensated to a gooddegree.

The four Wheatstone bridge contacts 191, 192, 193, and 194 connect thefour Wheatstone bridge resistors 181, 182, 183, and 184. This results ina Wheatstone bridge circuit 190 as in the embodiment of FIG. 2 whichtypically measures edgewise or flapwise strain of a rotor blade.

In order to measure edgewise and flapwise bending simultaneously, twoWheatstone bridge circuits 190 are provided which are adapted to measurerotor blade bending in substantially orthogonal directions. This istypically achieved by arranging the Wheatstone bridge circuits 190orthogonal to each other.

According to other embodiments, the Wheatstone bridge resistors 181,182, 183, and 184 may be connected to the Wheatstone bridge contacts191, 192, 193, and 194 in a different way. The Wheatstone bridgeresistors 181, 182, 183, and 184 may also be aligned along differentdirections, e.g. Wheatstone bridge resistors 181 and 182 may be alignedalong a horizontal direction while Wheatstone bridge resistors 183 and184 may be aligned along a vertical direction. According to furtherembodiments, it is possible to measure torsion moments of the rotorblade.

FIG. 4 shows a schematic longitudinal cross sectional view of a rotorblade 110 of a wind turbine 100 seen from a rotor blade root 150 to arotor blade tip. The rotor blade root 150 including strain sensors 160and temperature sensors 170 is closer to the observer of FIG. 4 than anairfoil portion 115 of rotor blade 110. Wind impinges rotor blade 110 ata leading edge 113 and leaves rotor blade 110 at a trailing edge 114.One part of the wind travels around rotor blade 110 on a pressure side112, another part of the wind travels around rotor blade 110 on asuction side 111 of rotor blade 110. Rotor blade 110 typically has around cross sectional shape at a flange at rotor blade root 150. Thiscross sectional shape changes its outline from a circle to a typicalairfoil shape as one travels from rotor blade root 150 to the airfoilportion 115 of rotor blade 110.

Rotor blade 110 includes one strain sensor 160 arranged at an innersurface of the rotor blade root 150 near pressure side 112 of rotorblade 110. Strain sensor 160 is adapted to measure a bending moment ofrotor blade 110 along a direction from blade root 150 to blade tip. Achord line of rotor blade 110 extends from leading edge 113 to trailingedge 114. Oscillations of rotor blade 110 in the direction of the chordline are sometimes called edge-wise oscillations. Oscillationsperpendicular to the direction of chord line between suction side 111and pressure side 112 are sometimes called flap-wise oscillations.Accordingly, directions along those oscillations are called edge-wiseand flap-wise directions.

In case of flap-wise oscillations of rotor blade 110, bending of rotorblade root 150 along a direction from rotor blade root to blade tip isvery large at portions near pressure side 112 and near suction side 111.If strain sensors 160 are located at those portions, flapwise bending ofrotor blade root 150 can be detected with high sensitivity.

As temperature-related effects are the most common causes of error instrain measurements, the strain sensor should be temperature calibratedsuch that the temperature dependency of the measured data is eliminated.

A temperature sensor 170 is arranged adjacent strain sensor 160.Temperature sensor 170 measures the temperature at the location ofstrain sensor 160. By doing this, a functional dependency of themeasured strain data on the local temperature may be obtained. Strainmay also be measured for various rotor angle positions, various pitchangle settings and for different temperatures. For that measured straindata, a regression analysis may be performed to compensate strain sensor160 with regard to temperature effects.

FIG. 5 shows a schematic longitudinal cross sectional view of a rotorblade 110 of a wind turbine 100 seen from a rotor blade root 150 to arotor blade tip. The rotor blade root 150 including strain sensors 160and temperature sensors 170 is closer to the observer of FIG. 5 than anairfoil portion 115 of rotor blade 110.

Rotor blade 110 includes two strain sensors 160 arranged at an innersurface of the rotor blade root 150. The strain sensors 160 are arrangedcircumferentially opposite to each other. One strain sensor 160 isarranged near suction side 111 of rotor blade 110; the other strainsensor 160 is arranged near pressure side 112 of rotor blade 110. Strainsensors 160 are adapted to measure a bending moment of rotor blade 110along a direction from blade root 150 to blade tip.

In case of flap-wise oscillations of rotor blade 110, bending of rotorblade root 150 is large at portions near pressure side 112 and nearsuction side 111. If strain sensors 160 are located at those portions,flapwise bending of rotor blade root 150 can be detected with highsensitivity.

For each strain sensor 160, there is provided a temperature sensor 170arranged adjacent to said strain sensor 160. The temperature at thelocation of a strain sensor 160 is measured by the temperature sensor170 arranged adjacent to that strain sensor 160. By doing this, afunctional dependency of the measured strain data on the localtemperature is obtained. Strain may also be measured for various rotorangle positions, various pitch angle settings and for differenttemperatures. For that measured strain data, a regression analysis maybe performed to compensate strain sensor 160 with regard to temperatureeffects.

According to some embodiments, each strain sensor 160 includes twostrain gauges. The in total four strain gauges of the two strain sensors160 are electrically connected to each other for forming a fullWheatstone bridge circuit 190 with the strain gauges being the resistorsof Wheatstone bridge circuit 190.

Using such a Wheatstone bridge circuit 190 in connection with rotorblade 110 of the embodiment of FIG. 5, flapwise bending of rotor blade110 may be measured with high accuracy.

FIG. 6 shows a schematic longitudinal cross sectional view of a rotorblade 110 of a wind turbine 100 seen from a rotor blade root 150 to arotor blade tip. The rotor blade root 150 including strain sensors 160and temperature sensors 170 is closer to the observer of FIG. 6 than anairfoil portion 115 of rotor blade 110.

Rotor blade 110 includes two strain sensors 160 arranged at an innersurface of the rotor blade root 150. Strain sensors 160 are arrangedcircumferentially opposite to each other. One strain sensor 160 isarranged facing leading edge 113 of the rotor blade 110; the otherstrain sensor 160 is arranged facing trailing edge 114 of rotor blade110. Strain sensors 160 are adapted to measure a bending moment of rotorblade 110 along a direction from blade root 150 to blade tip. As strainsensors 160 of the embodiment of FIG. 6 are arranged along the edge-wisedirection, strain sensors 160 are very sensitive to measure strain alongthe edge-wise direction.

According to the embodiment of FIG. 6, for each strain sensor 160 thereis a temperature sensor 170 arranged adjacent to said strain sensor 160.The temperature at the location of strain sensor 160 can be measured bythe temperature sensor 170. By doing this, a dependency of the measuredstrain data on the local temperature at the position of strain sensor160 is obtained.

FIG. 7 shows a schematic longitudinal cross sectional view of rotorblade 110 of wind turbine 100 seen from rotor blade root 150 to therotor blade tip. The rotor blade root 150 including strain sensors 160and temperature sensors 170 is closer to the observer of FIG. 7 than anairfoil portion 115 of rotor blade 110.

Rotor blade 110 includes four strain sensors 160 which are arranged atan inner surface of the rotor blade root 150 in the same plane. The twostrain sensors 160 at the top and bottom of FIG. 7 form the first strainsensor pair, while the two strain sensors 160 at the left and right ofFIG. 7 form the second strain sensor pair. The connection lines of thepairs of strain sensors 160 are orthogonal to each other. One strainsensor 160 is arranged near suction side Ill of rotor blade 110, onestrain sensor 160 is arranged near pressure side 112 of rotor blade 110,one strain sensor 160 is arranged facing leading edge 113 of the rotorblade 110 and one strain sensor 160 is arranged facing trailing edge 114of rotor blade 110. Strain sensors 160 are adapted to measure a bendingmoment of rotor blade 110 along a direction from blade root 150 to bladetip.

The first strain sensor pair at the top and bottom of FIG. 7 is alignedalong the flap-wise direction. Therefore, the first strain sensor pairis sensitive to measure strain along the flap-wise direction. The secondstrain sensor pair at the left and right of FIG. 7 is aligned along theedge-wise direction. Therefore, the second strain sensor pair issensitive to measure strain along the edge-wise direction.

According to some embodiments, each strain sensor 160 includes twostrain gauges. The in total eight strain gauges of the four strainsensors 160 form two full Wheatstone bridge circuits 190 with the straingauges being the resistors of the Wheatstone bridge circuits 190. Thefour strain gauges of the first strain sensor pair are electricallyconnected to each other for forming a first full Wheatstone bridgecircuit 190. The four strain gauges of the second strain sensor pair areelectrically connected to each other for forming a second fullWheatstone bridge circuit 190.

According to the embodiment of FIG. 7, for each strain sensor 160 thereis a temperature sensor 170 arranged adjacent to said strain sensor 160.What has been said about strain sensors 160 and temperature sensors 170in connection with the embodiments of FIGS. 5 and 6 also applies to theembodiment of FIG. 7.

In the following, two methods will be described which use so-called slowrolls of the wind rotor to calibrate strain sensors of rotor blades.Typically, slow rolls are characterized by the fact that the wind rotoris spinning slowly and almost no wind load is applied to the rotorblades 110. Therefore, substantially no bending of rotor blades due towind occurs during slow rolls. During slow rolls, the rotational speedof the electric generator is between 90 and 200 revolutions per minute(RPM). This corresponds to about 1 to 2 RPM of the rotor hub and a windspeed of about 2 to 3 meters per second (m/s). By contrast, the windturbine is typically operated with 12 to 15 RPM when producing electricenergy.

In a first method which is depicted in FIG. 8, a blade moment isdetermined using the factory settings of the used strain sensors. Thatblade moment is called measured blade moment M_(measured). For thedetermination of M_(measured) a fit function is used. M_(measured) isthen compared to a calculated blade moment M_(calc). M_(calc) can bedetermined to good accuracy, while the value of M_(measured) istypically less accurate as a fit function and factory settings are usedhereby. The fitting parameters of the fitting function are adjusteduntil M_(measured) and M_(calc) match within a predetermined errormargin. In a second method which is depicted in FIG. 10, strain data aremeasured for various blade positions and various temperatures.Subsequently, a regression analysis of the measured strain data withrespect to the temperature is performed, thus calibrating thetemperature dependency of the used strain sensors.

A gravitational or gravitationally induced blade moment is the bendingmoment acting on the rotor blade originating from the gravitationalforce. In case the rotor blade is pointing to the ground, no bendingmoment are acting within that rotor blade. Thus, the blade momentdepends on the angle of rotor blade 110 which is also called rotorposition, rotor blade 110 having a rotor position of 0° when pointingupwards.

The calculated gravitational blade moment M_(calc) is calculated usingthe formula

M _(calc) =m _(blade) ·g·L·sin α,

wherein m_(blade) is the mass of rotor blade 110, g is the standardgravity, which is the nominal acceleration due to gravity at the Earth'ssurface at sea level, α is the rotor position angle, and L is the lengthfrom the rotor axis to the center of gravitation (COG) of the rotorblade.

Additionally or alternatively, the calculated gravitational blade momentM_(calc) may be determined by simulations which typically may bemultibody simulations. Those simulations take into account the realorientation of the rotor blade with respect to the rotor axis, thestiffness of the used material of the rotor blade and so forth.

The gravitational blade moment can be measured during slow rolls withthe strain sensors provided in the rotor blades. Typically, the strainsensors are used in a Wheatstone bridge circuit as depicted in FIG. 2.In principle, the electrical signal of the strain sensors used in a fullWheatstone bridge circuit is linearly proportional to the strain.However, due to different CTEs of the used strain gauges of the strainsensors and the underlying material, there is a temperature dependentoffset and also a temperature dependent gain. The dependency of ameasured blade moment M_(measured) on strain may be expressed to a goodapproximation as follows:

M _(measured)=offset(T)+gain(T)·strain,

wherein offset(T) and gain(T) are temperature dependent functions. Theblade moment M_(measured) is a function of the rotor position α, andpitch angle. Ideally, M_(measured) should be independent of thetemperature. But in reality M_(measured) typically exhibits atemperature dependency which may be compensated for by the abovementioned methods of FIGS. 8 and 10. M_(measured) may also assumenegative values. To determine the offset value, M_(measured) is measuredat 90° and 270° rotor position, i.e. when the rotor blade is in the 3o'clock and 9 o'clock positions. These positions can be determinedaccurately since the measured strain will exhibit its maximum at thesepositions while zero-crossing at the 12 o'clock and 6 o'clock positions.The offset is determined as the sum of the M_(measured) values measuredat 90° and 270° divided by 2.

FIG. 8 illustrates a method for temperature calibration of a strainsensor 160 arranged at rotor blade 110 of wind turbine 100. The methodstarts by completing the installation of wind turbine 100 in step 300.In the next step 310, slow rolls are performed for pitch angles of 0°and 90° while blade moments are measured simultaneously using strainsensors 160.

In the next step 320, the measured blade moment M_(measured) isdetermined using the measured voltage values of the strain sensors andthe above mentioned formula. The first time step 320 is run, the factorysettings of the strain sensors are used.

In the following step 330, it is decided whether the measured blademoment M_(measured) equals the calculated blade moment M_(calc). Ifthose values do not match within a predetermined error margin, theoffset value and the gain value of the above mentioned fit function aremodified in step 340 and the method returns to step 320. Themodification of the offset and gain values in step 340 may be executedmanually or by a machine based algorithm. Now, in step 320, the measuredblade moment M_(measured) is determined again using the modified offsetand gain values. In case the measured blade moment M_(measured) and thecalculated blade moment M_(calc) match within a predetermined errormargin, the method continues with step 350 in which the determined gainand offset values are outputted.

FIG. 9 illustrates the relationship between measured blade moment curvesand the parameters ratio of span and difference of offset which are usedin embodiments described herein. In FIG. 9, rotor blade moments areplotted against the rotor position angle α. To get measurement curves asdepicted in FIG. 9, the pitch angles of two rotor blades are set to 65°and the pitch angle of the third rotor blade is set to 0°, 45°, and 90°,consecutively. The rotor position angle α in FIG. 9 refers to the thirdrotor blade. The blade moments of the third rotor blade is then measuredfor three 360° turns of the rotor using strain sensors. The plottedcurves in FIG. 9 are determined by averaging over three turns of themeasured rotor blade moments.

The solid curve in FIG. 9 is a typical measured curve using a strainsensor at the temperature T₀ which was used for the initial temperaturecalibration of the strain sensor at the factory. The dotted curve showsthe same measurement but for a different temperature T. Both curves aresinusoidal as the rotor blade moment is a sine function of the rotorblade angle α. One notices that the peak-to-peak amplitude of the twocurves is different, being 2M_(T) for the dotted curve measured attemperature T and being 2M₀ for the solid curve measured at temperatureT₀. Apart from the different peak-to-peak amplitudes, the sine of thesolid curve is centered at zero whereas the sine of the dotted curve iscentered at a different value. The difference of those center values iscalled the difference of offset value Δ which depends on the temperaturedifference of the two measured curves. The ratio of span R is the ratioof the peak-to-peak amplitudes of the curves measured at temperature T₀and T:

R=2M ₀/2M _(T).

Typically, the value of the ratio of span is about 1.

The aim of the temperature calibration is to have the same curve of theblade moment against rotor angle α for an arbitrary temperature. To thisend, one performs a transformation on the values of the curves measuredat temperatures T different from the temperature T₀ such that they aretransformed into the value of the curve measured at T₀. Therefore, therelationship between the calibrated blade moment M_(calibrated) and themeasured uncalibrated blade moment M_(uncalibrated) thus reads asfollows:

M _(calibrated) =R·(M _(uncalibrated)−Δ)

FIG. 10 illustrates a method for temperature calibration of strainsensor 160 according to another embodiment. The method starts byperforming slow rolls for pitch angles of 0°, 45°, and 90° whilesimultaneously measuring blade moments using strain sensors in step 400.In the next step 410, a minimum value and a maximum value for edgewiseand flapwise blade moments are determined. According to anotherembodiment, the minimum value and the maximum value is determined forthe edgewise or the flapwise blade moment. According to furtherembodiments, blade moments along arbitrary directions are determined.

In step 420, a ratio of span R and a difference of offset value Δaccording to the definitions given in connection with the description ofFIG. 9 are calculated. In step 430, it is determined whether the numberof data points is sufficiently large to perform reliable statisticalanalysis. Typically, a collection of about 30 data points measured atdifferent temperatures may be regarded as sufficiently large. Of course,smaller or larger threshold values can be chosen depending on therequired accuracy of the regression analysis in the following step. Incase the number of data points is larger than 30, the method continuesto step 440. In step 440, a regression analysis of the measured data isperformed with regard to blade temperature. In case the number of datapoints is not larger than 30, the method returns to step 400 where moredata points at different temperatures are measured.

According to some embodiments, no data is collected if the measuredtemperature of temperature sensors located at different locations differmore than 1 Kelvin (K). For temperature differences larger than 1 K, thedata may be considered not reliable enough or to produce inconsistentresults.

According to yet further embodiments, for cases with differenttemperatures at the strain sensors, a temperature calibration is alsopossible. For those cases, the regression takes those differenttemperatures into account and performs a calibration transformationwhich is similar to the transformation which was described in connectionwith FIG. 9 but has a different functional dependency as described inconnection with FIG. 9.

FIG. 11 shows a schematic longitudinal cross sectional view of a rotorblade root 150 of a wind turbine having two Wheatstone bridge circuitswhich are adapted to measure both edgewise and flapwise bending. Thefirst Wheatstone bridge circuit includes a first pair of strain sensors160, and the second Wheatstone bridge circuit includes a second pair ofstrain sensors 160. Both the first pair of strain sensors 160 and thesecond pair of strain sensors 160 are arranged circumferentiallyopposite to each other, the second pair of strain sensors 160 beingarranged circumferentially offset by about 90° to the first pair ofstrain sensors 160. Sometimes, it is not convenient to arrange thesecond pair of strain sensors 160 circumferentially offset by exactly90° to the first pair of strain sensors 160 due to space limitations. Inthese cases, the angle between the first pair of strain sensors 160 andthe second pair of strain sensors 160 is typically between 70° and 90°,preferably between 80° and 90°, or more preferably between 85° and 90°.

According to one embodiment, strain sensors 160 to the left and right ofFIG. 11 are adapted to measure edgewise bending, while strain sensors160 to the top and bottom of FIG. 11 are adapted to measure flapwisebending of rotor blade 110. Each strain sensor 160 includes two straingauges so that the embodiment of FIG. 11 includes eight strain gauges intotal. The strain sensors 160 depicted at the top and the bottom of FIG.11 thus form a full Wheatstone bridge circuit 190 as depicted in FIG. 2.The strain sensors 160 depicted at the left and the right of FIG. 11also form a full Wheatstone bridge circuit 190 as depicted in FIG. 2.For each strain sensor 160, there is provided a temperature sensor 170adjacent to that strain sensor 160.

According to another embodiment, temperature calibration of the strainsensors 160 includes controlling the temperature of a part of the rotorblade, e.g. of the rotor blade root 150, to a constant temperature andto measure the signal of the strain sensors 160 simultaneously forvarious controlled temperatures. A regression analysis may then beperformed using the measured data, and the result may be used tocompensate the temperature dependency of the strain sensors.

According to one embodiment, the temperature calibration of the strainsensor is performed in the factory with the rotor blade not beinginstalled on a wind turbine. Typically, the rotor blade is mountedforce-free. In such a case, there is no strain inside the rotor bladesuch that the formula mentioned above in connection with the descriptionof FIG. 8 simplifies to:

M _(measured)=offset(T).

In such a case, the temperature dependency of the offset value can becalibrated while the temperature dependent gain values are not measured.

According to a further embodiment, predetermined forces are exerted ontothe rotor blade. This way, it is possible to generate strain within therotor blade. This strain gives rise to an additional blade moment whichis equal to the strain multiplied by the temperature dependent gainfactor. In that case, one may also perform a temperature calibration ofthe gain value.

Typically, the temperature change of the offset value is about 10 timeslarger than the respective temperature change of the gain value.Therefore, the main distribution to the temperature dependency of theblade moment arises from the offset value. As the above mentionedforce-free mounting using predetermined extern forces is tedious, onetypically omits this measurement and performs only a temperaturecalibration of the offset value. In that case, the largest contributionto the error is calibrated. This means that a good calibration of thefactory settings is achieved.

FIG. 12 shows the rotor blade root of FIG. 11 wherein a heater 200 isarranged inside rotor blade root 150. For a temperature calibration, theheater 200 heats for some amount of time, e.g. until all temperaturesensors 170 measure the same temperature. At that point of time, thestrain sensors 160 measure the strain at their respective location atrotor blade root 150. That step is repeated for various temperatures.The collected data can then be used to perform a regression analysiswith which a temperature calibration of the strain sensors may beexecuted. In order to make sure that the temperature is constant at thelocations of strain sensors 160, the heater 200 may be placed in themiddle of rotor blade root 150.

FIG. 13 is a schematic perspective view of rotor blade root 150 of awind turbine 100. FIG. 13 shows the embodiment of FIG. 12 in aperspective view. Strain sensors 160 are arranged in a planeperpendicular to an axis from a center part of rotor blade root 150 torotor blade tip. Temperature sensors 170 are arranged next to the strainsensors 160 in the same plane as the strain sensors 160. The heater 200is also arranged in the same plane as strain sensors 160 and temperaturesensors 170.

FIG. 14 shows a schematic longitudinal cross sectional view of a rotorblade root 150 of a wind turbine 100 having two Wheatstone bridgecircuits 190 which are adapted to measure both edgewise and flapwisebending. What has been said about rotor blade root 150, strain sensors160 and temperature sensors 170 in connection with FIG. 12 also appliesto FIG. 14. The embodiment of FIG. 14 does not use the heater 200 shownin FIGS. 12 and 13. Instead, a heating mat 210 is used which is arrangedon part of a surface of rotor blade 110. As seen in a cross sectionalview of FIG. 14 rotor blade root 150 is enshrouded by heating mat 210.What has been said about the temperature calibration in connection withFIG. 12 also applies to FIG. 14.

FIG. 15 is a schematic perspective view of a rotor blade root of a windturbine 100. FIG. 15 shows the embodiment of FIG. 14 in a perspectiveview. Regarding an axis from a center part of rotor blade root 150 torotor blade tip, heating mat 210 which is wrapped around rotor bladeroot 150 is centered in the plane in which strain sensors 160 andtemperature sensors 170 are arranged.

In further embodiments, heater 200 and heating mat 210 may be exchangedwith a cooling device. In particular, these cooling devices may beadapted to cool down the rotor blade portion including the strainsensors. Thus, the temperature dependency of the strain sensors may alsobe verified for lower temperatures, e.g. below freezing point. Accordingto some embodiments, however, it should be noted that the temperaturedependency of the strain sensors may be relatively low for temperaturesbelow 15° C.

FIG. 16 illustrates yet a further method for temperature calibration ofa strain sensor 160 arranged at a rotor blade 110 of a wind turbine 100.The method may be performed using a rotor blade equipped with a heatingdevice according to any of the embodiments of the FIGS. 12 to 15.

The method starts in step 500 by controlling the temperature of part ofrotor blade 110 in which the strain sensor 160 is located. According tosome embodiments, this is done by a heating mat on the outside of therotor blade root. According to other embodiments, this is done by aheater arranged inside the rotor blade root.

In step 510, the strain at the location of strain sensor 160 and thetemperature at the location of strain sensor 160 are measured.

Although only one strain sensor 160 and only one temperature sensor 170are shown in FIG. 16 exemplarily, it will be understood by the skilledreader that also a plurality of strain sensors 160 and a plurality oftemperature sensors 170 may be used in some embodiments. The temperaturemeasurement of the temperature sensor 170 may be time shifted from thestrain measurement. The temperature sensors 170 may also be located atanother location as the strain sensors 160. According to otherembodiments, the number of temperature sensors 170 is not equal to thenumber of strain sensors 160. In particular, the number of temperaturesensors 170 may be smaller than the number of strain sensors 160.

In the next step 520, it is determined whether the number of measuredtemperature data points is sufficient. This determination may be done bycomparison with a predetermined number of data points to be measured.According to some embodiments, the data points are equally distributedwithin a temperature interval between two temperatures. The method thenstarts with either the lowest or the largest temperature and thencontinues to measure towards the other extreme temperature point.According to further embodiments, this determination is done bystatistical calculations.

In case the number of measured temperature data points is notsufficient, the method continues to step 530, while if the number ofmeasured temperature data points is sufficient, the method continues tostep 540.

In step 530, the controlled temperature is varied to a differenttemperature. After step 530, the method returns to step 510 in which thestrain at the location of strain sensor 160 and the temperature at thelocation of strain sensor 160 are measured.

If it was determined in step 520 that the number of measured temperaturedata points is sufficient, the method continues to step 540. In step 540a temperature dependency of the strain sensor from the measured data isdetermined. According to some embodiments, this determination is done byregression analysis methods. According to other embodiments, atemperature dependency of the measured signal of the strain sensor isdetermined. This determination may also be determined by regressionanalysis.

In the last step of the method, step 550 the strain sensor is calibratedbased on the determined temperature dependency. After step 550 themethod ends.

According to some embodiments, no data is collected if the measuredtemperature of temperature sensors located at different locations differmore than 1 Kelvin (K).

Typically, this method of the embodiment of FIG. 16 is performed with asingle rotor blade which is not yet installed in a wind turbine. Hereby,the temperature calibration of the strain sensor is done beforeinstallation of the wind turbine. As the rotor blades are not yetinstalled, this calibration may be done more thoroughly than afterinstallation of the wind turbine. Therefore, one readily may use thewind turbine with a complete set of calibrated rotor blades which isadvantageous.

This written description uses examples, including the best mode, toenable any person skilled in the art to make and use the describedsubject-matter. While various specific embodiments have been disclosedin the foregoing, those skilled in the art will recognize that thespirit and scope of the claims allows for equally effectivemodifications. Especially, mutually non-exclusive features of theembodiments described above may be combined with each other. Thepatentable scope is defined by the claims, and may include suchmodifications and other examples that occur to those skilled in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal language of theclaims.

1. A rotor blade comprising: a first strain sensor arranged at a surfaceof the rotor blade; and, a first temperature sensor arranged adjacent tothe first strain sensor.
 2. The rotor blade according to claim 1,further comprising a second strain sensor arranged at the surface of therotor blade circumferentially opposite to the first strain sensor; and,a second temperature sensor arranged adjacent to the second strainsensor.
 3. The rotor blade according to claim 2, wherein each strainsensor comprises two strain gauges, each strain gauge being adapted tomeasure a bending moment along a direction from blade root to blade tip;and, the four strain gauges of the first and second strain sensors areelectrically connected to each other to form a full Wheatstone bridgecircuit.
 4. The rotor blade according to claim 2, further comprisingthird and fourth strain sensors arranged at the surface of the rotorblade, the third strain sensor being arranged circumferentially oppositeto the fourth strain sensor and circumferentially offset by about 90° tothe first strain sensor; third and fourth temperature sensors, the thirdtemperature sensor arranged adjacent to the third strain sensor and thefourth temperature sensor arranged adjacent to the fourth strain sensor.5. The rotor blade according to claim 2, wherein the first strain sensoris arranged at a leading edge of the rotor blade; and, the second strainsensor is arranged at a trailing edge of the rotor blade.
 6. The rotorblade according to claim 2, wherein the first strain sensor is arrangedat a suction side of the rotor blade; and, the second strain sensor isarranged at a pressure side of the rotor blade.
 7. A method fortemperature calibration of a strain sensor arranged at a rotor blade ofa wind turbine, the method comprising: operating the wind turbine in amode in which substantially no bending of the rotor blade due to windoccurs; repeatedly measuring gravitationally induced bending moments ofthe rotor blade for a plurality of temperatures measured at the locationof the strain sensor; determining a temperature dependency of the strainsensor from said measured data; calibrating the strain sensor based onthe determined temperature dependency of the strain sensor such that thetemperature dependency of the strain sensor is compensated.
 8. Themethod according to claim 7, wherein measuring gravitationally inducedbending moments of the rotor blade comprises: measuring bending momentsof the rotor blade for a plurality of azimuth positions of the rotorblade and a plurality of pitch angles of the rotor blade.
 9. The methodaccording to claim 7, further comprising: calculating a gravitationallyinduced bending moment of the rotor blade based on the physicalproperties of the rotor blade, and a rotor azimuth position; comparingthe calculated bending moment of the rotor blade with the measuredbending moment for said rotor azimuth position; calibrating the strainsensor by setting a correction value thus that the measured bendingmoment equals the calculated bending moment.
 10. The method according toclaim 7, further comprising: determining a minimum value and a maximumvalue of the bending moment of the rotor blade; determining a ratio ofspan and a difference of offset values; determining if the number ofdata points is sufficient; determining a calibrated bending moment whichis equal to a product of the ratio of span value and a difference of thenon-calibrated bending moment and the difference of offset value. 11.The method according to claim 10, wherein the minimum value and themaximum value of the bending moment of the rotor blade, and the bendingmoment of the rotor blade are determined for a flapwise direction and anedgewise direction.
 12. The method according to claim 7, furthercomprising determining a functional dependency of the bending momentsmeasured by the strain sensor on the temperature by a regressionanalysis.
 13. A method for temperature calibration of a strain sensorarranged at a rotor blade of a wind turbine, the method comprising:controlling a temperature of a part of the rotor blade in which saidstrain sensor is located; measuring a strain using the strain sensor;measuring the temperature at the location of the strain sensor; varyingthe controlled temperature and repeating the strain and temperaturemeasurements at a different temperature; determining a temperaturedependency of the strain sensor from said measured data; and,calibrating the strain sensor based on the determined temperaturedependency of the strain sensor such that the temperature dependency ofthe strain sensor is compensated.
 14. The method according to claim 13,wherein the temperature is controlled by means of a heating mat arrangedon a part of a surface of said rotor blade.
 15. The method according toclaim 14, wherein the heated part of the surface of the rotor blade islarger than the joining area of the rotor blade and the strain sensor.16. The method according to claim 13, wherein the temperature iscontrolled by means of a heating fan arranged inside the rotor blade.17. The method according to claim 13, wherein the temperature iscontrolled by means of a vapor compression refrigeration system adaptedto cool a part of a surface of the rotor blade.
 18. The method accordingto claim 13, further comprising: thermally insulating thetemperature-controlled part of the rotor blade from a further part ofthe rotor blade in which the temperature is not controlled.
 19. Themethod according to claim 13, wherein the temperature is controlledbetween about −20° C. to about +50° C.
 20. The method according to claim13, further comprising: increasing the temperature of a part of therotor blade from an initial temperature to a final temperature in a stepwise manner; measuring, in every step, a bending moment of the rotorblade using the strain sensor; measuring, in every step, the temperatureat the position of the strain sensor; determining a functionaldependency between the bending moment and the measured temperature.